Cities are critical actors in reducing global greenhouse gas (GHG) emissions; the level of knowledge, skills, and capacity, especially regarding innovative project planning such as sustainable energy (SE) projects, is fundamental for cities to meet their untapped potential. A range of indicator-based frameworks have been developed, focusing on general city economic competitiveness, yet these are often disconnected from the actual city operation and processes relevant to the investment cycle of SE projects. This paper proposes an integrated methodology for developing a City Capability Assessment Framework focusing on city planning, financing, and implementation capacity for SE projects. A principal component analysis (PCA) was applied to the results of a European city-survey to uncover areas highly relevant for the successful implementation of local SE projects. The insights gained aid policymakers to better understand implementation barriers, leading to the development of more effective actions and policy interventions towards the implementation of SE projects. The proposed framework can further enhance cities’ efforts with the implementation of local SE projects and guide potential investors to properly evaluate city capabilities and potential, when choosing local investment projects.

Local governments increasingly adopt climate and energy strategies addressing both mitigation and adaptation objectives, yet these domains are often treated separately, limiting integrated planning. This study develops a structured Climate–Energy Action Library to support more coherent local decision-making. The library was constructed through a systematic review and harmonisation of actions from European Sustainable Energy and Climate Action Plans (SECAPs), international repositories, and related frameworks, resulting in a taxonomy of 171 actions grouped into thematic bundles and policy categories. The methodology enables the identification of potential synergies among measures, and revealing consistent cross-sector interaction patterns. The strongest interaction potential occurs when technical measures are combined with enabling governance actions, including policy instruments, planning frameworks, and capacity-building. Cross-sectoral synergies are evident in building retrofit programmes linked with heat-stress adaptation and in nature-based solutions contributing to mitigation, urban cooling, and ecosystem services. These findings indicate that governance and ecosystem-based measures often enhance the effectiveness of sector-specific interventions. The proposed library provides a practical analytical reference for municipalities, supporting the design and evaluation of integrated climate strategies and helping bridge the persistent separation between mitigation and adaptation in local climate governance.